Method for forming electroless plating film and film formation device

ABSTRACT

This disclosure provides a method for forming a plating film capable of suppressing deterioration of a plating solution, and a film formation device. The embodiment is a method for forming a metal plating film on a metal substrate by a substitution-type electroless plating method. The method includes bringing a porous film containing an electroless plating solution into contact with a surface of the metal substrate, and the porous film has an anionic group.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims priority from Japanese patent application JP 2019-122302 filed on Jun. 28, 2019, the entire content of which is hereby incorporated by reference into this application.

BACKGROUND Technical Field

This disclosure relates to a method for forming an electroless plating film and a film formation device.

Background Art

Generally, a method for plating by reducing metal ions in a plating solution is roughly divided into an electroplating method using an external current and an electroless plating method not using an external electricity. The latter electroless plating method is further roughly divided into (1) a substitution-type electroless plating method where metal ions in a solution are reduced by electrons, which are released by dissolution of an object to be plated, and deposited on the object to be plated, and (2) an autocatalytic reduction-type electroless plating method where metal ions in a solution are deposited as a metal film by electrons released when a reducing agent contained in the solution is oxidized. Since the electroless plating method ensures uniform deposition even on a surface with a complicated shape, the electroless plating method is widely used in many fields.

In the substitution-type electroless plating, a difference in ionization tendency between a metal in a plating solution and an underlying metal is used to form a metal plating film. For example, in a gold plating method, when a substrate on which an underlying metal is formed is immersed in a plating solution, the underlying metal having a high ionization tendency becomes ions to be dissolved in the plating solution, and gold ions in the plating solution are deposited on the underlying metal as a metal to form a gold plating film.

For example, JP 2005-307309 A discloses a substitution-type electroless plating solution using the substitution-type electroless plating method. JP 2005-307309 A discloses an electroless gold plating solution to form a gold film on an electroless nickel plating film, and the electroless gold plating solution contains (a) a water-soluble gold compound, (b) a conductive salt containing an acidic substance having an acid dissociation constant (pKa) of 2.2 or less, and (c) an oxidation inhibitor containing a heterocyclic aromatic compound having two or more nitrogen atoms in a molecule as an essential component.

SUMMARY

As described above, in the substitution-type electroless plating method, the underlying metal becomes ions to be dissolved in the plating solution. Therefore, repeatedly performing the plating process causes progress of deterioration of the plating solution due to the dissolved underlying metal. Accordingly, it has been desired to provide a technique capable of suppressing the deterioration of the plating solution regardless of the repeated use of the plating solution.

The present disclosure provides a method for forming a plating film capable of suppressing deterioration of a plating solution, and a film formation device.

The present inventors have intensively studied to solve the above-described problem and found that, unexpectedly, the use of a porous film having an anionic group ensures the formation of a plating film while suppressing the deterioration of a plating solution, thereby arriving at the present disclosure.

Exemplary aspects of the embodiment are described as follows.

(1) A method for forming a metal plating film on a metal substrate by a substitution-type electroless plating method, the method comprising bringing a porous film into contact with a surface of the metal substrate, the porous film having an anionic group and containing an electroless plating solution. (2) The method according to (1) wherein the bringing includes reducing metal ions derived from the electroless plating solution contained in the porous film to deposit the metal plating film on the surface of the metal substrate. (3) The method according to (1) or (2) wherein the anionic group comprises at least one kind selected from the group consisting of sulfonate group, thiosulfonate group, carboxy group, phosphate group, phosphonate group, hydroxy group, cyano group, or thiocyano group. (4) The method according to any one of (1) to (3) wherein the porous film is a solid electrolyte membrane having an ionic conductivity. (5) The method according to (4) wherein the solid electrolyte membrane comprises a fluorine-based resin having sulfonate group. (6) The method according to (5) wherein the solid electrolyte membrane has an equivalent weight (EW) of 850 to 950 g/mol. (7) The method according to any one of (1) to (6) wherein the electroless plating solution comprises an electroless gold plating solution. (8) The method according to (7) wherein the electroless gold plating solution comprises at least a gold compound and a complexing agent. (9) The method according to (8) wherein the gold compound comprises a non-cyanide gold salt, and the complexing agent comprises a non-cyanide complexing agent. (10) The method according to (9) wherein the non-cyanide gold salt comprises a gold sulfite salt. (11) The method according to (9) or (10) wherein the non-cyanide complexing agent comprises a sulfite salt. (12) The method according to (10) or (11) wherein the anionic group comprises sulfonate group. (13) The method according to (9) wherein the non-cyanide complexing agent comprises mercaptosuccinic acid. (14) The method according to (13) wherein the anionic group comprises carboxy group. (15) The method according to any one of (1) to (14) wherein the metal substrate comprises nickel or a nickel alloy. (16) The method according to any one of (1) to (15) wherein the electroless plating solution is supplied from a plating solution chamber that houses the electroless plating solution. (17) The method according to (16) wherein the plating solution chamber is disposed in contact with the porous film. (18) A film formation device for forming a metal plating film on a metal substrate by a substitution-type electroless plating method, the film formation device comprising:

a porous film having an anionic group;

a plating solution chamber disposed in contact with the porous film, the plating solution chamber housing an electroless plating solution; and

a pressing unit that brings the porous film into contact with the metal substrate by relatively pressing the plating solution chamber and the metal substrate.

This disclosure can provide the method for forming the plating film capable of suppressing the deterioration of the plating solution, and the film formation device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view for describing an exemplary configuration of a film formation device according to the embodiment;

FIG. 2 is a schematic cross-sectional view for describing a state of a film formation device 1 illustrated in FIG. 1 during film formation; and

FIG. 3 is a graph illustrating a relationship between equivalent weights (EW) and coverages when gold plating films are formed using eight kinds of solid electrolyte membrane having different equivalent weights manufactured in Example 3.

DETAILED DESCRIPTION

An aspect of the embodiment is a method for forming a metal plating film on a metal substrate by a substitution-type electroless plating method. The method includes a step of bringing a porous film into contact with a surface of the metal substrate, and the porous film has an anionic group and contains an electroless plating solution.

In the method (method for forming the plating film) according to the embodiment, by bringing the porous film, which contains the electroless plating solution and has the anionic group, into contact with the metal substrate, a metal of the metal substrate becomes ions to be dissolved in the electroless plating solution, and the metal ions derived from the electroless plating solution are reduced to be deposited on the surface of the metal substrate, thereby forming the plating film. The metal ions dissolved in the electroless plating solution, which are derived from the metal substrate, are captured by the anionic group in the porous film. Therefore, the deterioration of the plating solution due to the metal derived from the metal substrate can be suppressed.

The method according to the embodiment can also provide an effect that the plating film can be formed using a small amount of the plating solution. That is, in a conventional electroless plating method, generally, the plating film is formed on an object to be plated by immersing the object to be plated in the plating solution. For immersing the object to be plated in the plating solution, a relatively large amount of the plating solution needs to be used. Meanwhile, since the usage of the plating solution in the method according to the embodiment is substantially only the amount to be impregnated to the porous film, the amount is smaller than the conventional amount used for immersing the metal substrate. Therefore, the method according to the embodiment can form the plating film using a small amount of the plating solution.

The following describes the embodiment in detail.

(Metal Substrate)

The metal substrate as the object to be plated is not specifically limited. For example, when a gold plating film is formed on the metal substrate, a metal constituting the metal substrate is not specifically limited insofar as the metal has an ionization tendency higher than that of gold, and the metal can include, for example, copper, nickel, cobalt, palladium, or an alloy containing at least two kinds of them. In one embodiment, the metal substrate is nickel or a nickel alloy. When the metal substrate is nickel or the nickel alloy, the gold plating film can be easily formed by the substitution-type electroless plating method.

The metal substrate can have any shape. The shape of the metal substrate includes, for example, a plate shape such as a flat plate shape or a curved plate shape, a rod shape, or a spherical shape. The object to be plated may be an object on which fine processing, such as a groove and a hole, is performed, and may be, for example, a wiring for an electronic industrial component, such as a printed wiring board, an ITO substrate, and a ceramic IC package substrate. The metal substrate may be a plating film formed on a resin product, a glass product, or a product such as a ceramic component.

(Porous Film)

The porous film has an anionic group. When the porous film has the anionic group, the anionic group can capture the metal ions dissolved from the metal substrate. Therefore, the deterioration of the electroless plating solution due to the metal ions (for example, nickel ions) derived from the metal substrate can be suppressed. Since the porous film having the anionic group is hydrophilic, the wettability of the porous film is improved. Therefore, since the porous film having the anionic group is easily wettable by the electroless plating solution, the electroless plating solution can be uniformly spread on the metal substrate. Consequently, the porous film having the anionic group provides an effect that the uniform metal plating film can be formed.

While the anionic group is not specifically limited, the anionic group includes at least one kind selected from, for example, sulfonate group, thiosulfonate group (—S₂O₃H), carboxy group, phosphate group, phosphonate group, hydroxy group, cyano group, or thiocyano group. These anionic groups can capture metal ions having positive electric charges. These anionic groups can give the hydrophilicity to the porous film. The anionic group may comprise sulfonate group or carboxy group. Especially, the anionic group may comprise sulfonate group (sulfo group) because nickel ions can be effectively captured.

As a material of the porous film having the anionic group, an anionic polymer can be used. That is, the porous film having the anionic group includes the anionic polymer. The anionic polymer has the anionic group (for example, the sulfonate group, the thiosulfonate group, the carboxy group, the phosphate group, the phosphonic acid group, the hydroxy group, the cyano group, or the thiocyano group described above). The anionic polymer may have one kind of the anionic groups alone, or may have two kinds or more of the anionic group in combination. The anionic group may be the sulfonate group.

While the anionic polymer is not specifically limited, the anionic polymer can include, for example, a polymer containing a monomer having the anionic group.

Representatively, the anionic polymer includes, for example, a polymer having carboxyl group [for example, a (meth)acrylic acid polymer (for example, a copolymer of (meth)acrylic acid and another copolymerizable monomer such as poly(meth)acrylic acid), or a fluorine-based resin having carboxyl group (perfluorocarboxylic acid resin)], a styrene resin having sulfonate group [for example, polystyrene sulfonic acid], and a sulfonated polyarene ether resin [for example, sulfonated polyether ketone resin, sulfonated polyethersulfone resin].

The porous film may include a solid electrolyte membrane having ionic conductivity. The solid electrolyte membrane internally has an ion cluster structure, and the plating solution is impregnated to inside the ion cluster structure. Since the metal ions, such as gold ions, in the plating solution is coordinated to the anionic group in the solid electrolyte membrane, the metal ions are effectively spread in the solid electrolyte membrane. Therefore, the use of the solid electrolyte membrane ensures the formation of the uniform plating film.

The solid electrolyte membrane has a porous structure (that is, ion cluster structure), and pores of the porous structure are very small, having an average pore diameter of, for example, 0.1 μm or more and 100 μm or less. By applying a pressure, the electroless plating solution can be immersed into the solid electrolyte membrane. While the solid electrolyte membrane can include, for example, a fluorine-based resin, such as Nafion (registered trademark) manufactured by DuPont, a hydrocarbon resin, a polyamic acid resin, and a resin having an ion exchange function, such as Selemion (CMV, CMD, CMF series) manufactured by AGC Inc., the solid electrolyte membrane is not specifically limited to them. The solid electrolyte membrane may be the fluorine-based resin having the sulfonate group. The fluorine-based resin having the sulfonate group has a hydrophobic part of a fluorinated carbon skeleton and a hydrophilic part of a side chain part having the sulfonate group, and these parts form the ion cluster. The metal ions in the plating solution impregnated to inside the ion cluster are coordinated to the sulfonate group of the solid electrolyte membrane, and uniformly spread in the solid electrolyte membrane. Since the solid electrolyte membrane having the sulfonate group is easily wettable by the plating solution because of high hydrophilicity and excellent wettability, the plating solution can be uniformly spread on the metal substrate. Therefore, the use of the fluorine-based resin having the sulfonate group ensures the formation of the uniform plating film. The use of the fluorine-based resin having the sulfonate group increases dielectric polarization generated at a diffusion layer between the solid electrolyte membrane and the metal substrate due to Maxwell-Wagner effect, thus ensuring high speed transport of the metal ions. Such a fluorine-based resin is available as, for example, a series of a product name “Nafion” from DuPont.

The equivalent weight (EW: Equivalent Weight) of the solid electrolyte membrane may be 850 g/mol or more and 950 g/mol or less, and may be 874 g/mol or more and 909 g/mol or less. The respective upper limit values and lower limit values of these numerical ranges can be freely combined to specify an appropriate range. Here, the equivalent weight means a dry mass of the solid electrolyte membrane per equivalent of an ion exchange group. When the equivalent weight of the solid electrolyte membrane is in this range, the uniformity of the metal plating film can be improved.

While an adjustment method of the equivalent weight of the solid electrolyte membrane is not specifically limited, for example, in the case of a perfluorocarbon sulfonic acid polymer, the adjustment can be performed by changing a polymerization ratio between a fluorinated vinyl ether compound and a fluorinated olefin monomer. Specifically, for example, by increasing the polymerization ratio of the fluorinated vinyl ether compound, the equivalent weight of the solid electrolyte membrane to be obtained can be decreased. The equivalent weight can be measured using a titration method.

A film thickness of the porous film may be 10 μm or more and 200 μm or less, and may be 20 μm or more and 160 μm or less. The respective upper limit values and the lower limit values of these numerical ranges can be freely combined to specify an appropriate range. When the film thickness of the solid electrolyte membrane is 10 μm or more, the solid electrolyte membrane is not easily broken and has an excellent durability. When the film thickness of the solid electrolyte membrane is 200 μm or less, the pressure necessary for causing the electroless plating solution to pass through the solid electrolyte membrane can be reduced.

A water contact angle of the porous film may be 15° or less, may be 13° or less, and may be 10° or less. When the water contact angle of the porous film is within this range, the wettability of the porous film can be improved.

(Electroless Plating Solution)

The electroless plating solution used in the embodiment is what is called a substitution-type electroless plating solution. The electroless plating solution contains, for example, a metal compound and a complexing agent, and may contain an additive as necessary. The additive includes, for example, a pH buffer or a stabilizer. A commercially available plating solution may be used. The electroless plating solution is, for example, an electroless gold plating solution. The following describes the electroless gold plating solution in detail.

The electroless gold plating solution contains at least a gold compound and a complexing agent, and may contain an additive as necessary.

While the gold compound is not specifically limited, the gold compound includes, for example, a cyanide gold salt or a non-cyanide gold salt (cyanide-free-type gold salt). The cyanide gold salt includes, for example, gold cyanide, gold potassium cyanide, gold sodium cyanide, ammonium gold cyanide, or the like. The non-cyanide gold salt includes, for example, a gold sulfite salt, a gold thiosulfate salt, a chloroaurate, a gold thiomalate, or the like. One kind of gold salt may be used alone, or two or more kinds thereof may be used in combination. As the gold salt, from the aspect of handling, environment, and toxicity, the non-cyanide gold salt may be used, and the gold sulfite salt among the non-cyanide gold salts may be used. The gold sulfite salt can include, for example, ammonium gold sulfite, potassium gold sulfite, gold sodium sulfite, methanesulfonic acid gold salt, or the like.

The content of the gold compound in the electroless gold plating solution may be in a range of 0.5 g/L or more and 2.5 g/L or less as gold, and may be in a range of 1.0 g/L or more and 2.0 g/L or less. The respective upper limit values and the lower limit values of these numerical ranges can be freely combined to specify an appropriate range. When the gold content is 0.5 g/L or more, a gold deposition reaction can be improved. When the gold content is 2.5 g/L or less, stability of the plating solution can be improved.

The complexing agent provides an effect to stably complex gold ions (Au⁺) to decrease the occurrence of a disproportionation reaction of Au⁺ (3Au⁺→Au³⁺+2Au), thereby improving the stability of the liquid. One kind of the complexing agent may be used alone, or two or more kinds may be used in combination.

The complexing agent includes, for example, a cyanide complexing agent or a non-cyanide complexing agent. The cyanide complexing agent includes, for example, sodium cyanide or potassium cyanide. The non-cyanide complexing agent includes, for example, sulfite, thiosulfate, thiomalate, thiocyanate, mercaptosuccinic acid, mercaptoacetic acid, 2-mercaptopropionic acid, 2-aminoethanethiol, 2-mercaptoethanol, glucose cysteine, 1-thioglycerol, sodium mercaptopropane sulfonate, N-acetyl methionine, thiosalicylic acid, ethylenediaminetetraacetic acid (EDTA), and pyrophosphoric acid. As the complexing agent, from the aspect of handling, environment, and toxicity, the non-cyanide complexing agent may be used, and the sulfite among the non-cyanide complexing agent may be used.

The content of the complexing agent in the electroless gold plating solution may be 1 g/L or more and 200 g/L or less, and may be 20 g/L or more and 50 g/L or less. The respective upper limit values and the lower limit values of these numerical ranges can be freely combined to specify an appropriate range. When the content of the complexing agent is 1 g/L or more, a gold complexing ability is increased to improve the stability of the plating solution. When the content of the complexing agent is 200 g/L or less, generating of recrystallization in the plating solution can be suppressed.

The electroless gold plating solution can contain an additive as necessary. The additive includes, for example, a pH buffer or a stabilizer.

The pH buffer can adjust a deposition rate to a desired value, and can keep pH of the plating solution constant. One kind of the pH buffer may be used alone, or two or more kinds may be used in combination. The pH buffer includes, for example, phosphate, acetate, carbonate, borate, citrate, or hydrosulfate.

The pH of the electroless gold plating solution may be 5.0 or more and 8.0 or less, may be 6.0 or more and 7.8 or less, or may be 6.8 or more and 7.5 or less. The respective upper limit values and the lower limit values of these numerical ranges can be freely combined to specify an appropriate range. When the pH is 5.0 or more, the stability of the plating solution tends to be improved. When the pH is 8.0 or less, corrosion of the metal substrate as the underlying metal can be suppressed. The pH can be adjusted by adding, for example, potassium hydroxide, sodium hydroxide, and ammonium hydroxide.

The stabilizer can improve the stability of the plating solution. The stabilizer includes, for example, a thiazole compound, a bipyridyl compound, or a phenanthroline compound.

A commercially available electroless gold plating solution may be used. The commercial product includes, for example, EPITHAS TDS-25, TDS-20 (C. Uyemura & Co., Ltd.), or FLASH GOLD (manufactured by OKUNO CHEMICAL INDUSTRIES CO., LTD.).

The embodiment includes an embodiment where the electroless gold plating solution contains gold sulfite salt as a gold compound and/or sulfite salt as a complexing agent, and the porous film is a solid electrolyte membrane having the sulfonate group as an anionic group. The gold sulfite salt and the sulfite salt are easily impregnated to the solid electrolyte membrane having the sulfonate group, and further, the gold ion is coordinated to the sulfonate group to be effectively spread in the solid electrolyte membrane. Therefore, the gold ion is sufficiently supplied to the film formation part to ensure uniformly forming the plating film.

The embodiment includes an embodiment where the electroless gold plating solution contains a carboxyl group-containing compound as a complexing agent, and the porous film is a solid electrolyte membrane having carboxy group as an anionic group. The carboxyl group-containing compound includes, for example, mercaptosuccinic acid, acetylcysteine, or cysteine in addition to the compounds listed above. The carboxyl group-containing compound can form a stable complex with gold ions. By combining the solid electrolyte membrane having carboxyl group and the gold plating solution containing the carboxyl group-containing compound, the plating solution can be stably kept at a mild acidity, thereby ensuring the formation of the uniform plating film. The carboxyl group-containing compound is easily impregnated to the solid electrolyte membrane having carboxy group, and further, the gold ion is coordinated to the carboxyl group to be effectively spread in the solid electrolyte membrane. Therefore, the gold ion is sufficiently supplied to the film formation part to ensure uniformly forming the plating film.

(Film Formation Device and Plating Film Formation Step)

An aspect of the embodiment is a film formation device to form a metal plating film on a metal substrate by a substitution-type electroless plating method. The film formation device includes a porous film having an anionic group, a plating solution chamber that is disposed in contact with the porous film and houses an electroless plating solution, and a pressing unit that relatively presses the plating solution chamber and the metal substrate to bring the porous film into with the metal substrate.

The film formation device according to the embodiment can supply the electroless plating solution to the metal substrate passing through the porous film by pressing the porous film onto the metal substrate by the pressing unit. A metal of the metal substrate becomes ions to be dissolved in the electroless plating solution, the metal ions derived from the electroless plating solution are reduced to be deposited on the surface of the metal substrate, thereby forming the plating film. The metal ions, derived from the metal substrate, dissolved in the electroless plating solution are captured by the anionic group in the porous film. Therefore, the deterioration of the plating solution can be suppressed. Entering of the metal derived from the metal substrate to the plating solution chamber can be avoided, thereby ensuring the suppression of the deterioration of the plating solution in the plating solution chamber.

The film formation device according to the embodiment provides an effect that the plating film can be formed using a small amount of the plating solution. That is, in a conventional electroless plating method, generally, the plating film is formed on an object to be plated by immersing the object to be plated in the plating solution. For immersing the object to be plated in the plating solution, a relatively large amount of the plating solution needs to be used. Meanwhile, since the usage of the plating solution in the film formation device according to the embodiment is substantially only the amount to be impregnated to the porous film, the amount is smaller than the conventional amount used for immersing the metal substrate. Therefore, the film formation device according to the embodiment can form the plating film using a small amount of the plating solution.

FIG. 1 is a schematic cross-sectional view illustrating an exemplary configuration of a film formation device 1 according to the embodiment. As illustrated in FIG. 1, the film formation device 1 according to the embodiment is a device to form a metal plating film by the electroless plating method, and reduces metal ions derived from an electroless plating solution to deposit a metal, thereby forming the metal plating film on a surface of a metal substrate B.

The film formation device 1 includes a porous film (for example, solid electrolyte membrane) 13 having an anionic group, a plating solution chamber 15 that is disposed in contact with the porous film 13 and houses an electroless plating solution L, and a pressing unit 18 that relatively presses the plating solution chamber 15 and the metal substrate B to bring the porous film 13 into contact with the metal substrate B. The plating solution chamber 15 is formed of a housing 12. The porous film 13 is installed to an opening end of the housing 12, and houses the electroless plating solution L in the plating solution chamber 15 with the housing 12. That is, the housing 12 includes the plating solution chamber 15 for housing the electroless plating solution L, and includes an opening 12 a on the metal substrate B side of the plating solution chamber 15. The porous film 13 is installed to the housing 12 so as to seal the opening 12 a of the housing 12. The film formation device 1 includes a placement table 40 for placing the metal substrate B. The film formation device 1 includes the pressing unit 18 on the top of the housing 12 via a cushioning member 19, such as a spring. The pressing unit 18 can be, for example, a hydraulic or pneumatic cylinder. Accordingly, the porous film 13 is pressed onto the surface of the metal substrate B, thereby ensuring the formation of the metal plating film. The cushioning member 19 ensures gradually pressing the porous film 13 onto the surface of the metal substrate B. The film formation device 1 does not require an electrode, such as a positive electrode.

While the material of the housing 12 is not specifically limited, the material can be, for example, a metallic material or a resin material. Since the electroless plating solution L is housed in a space formed by the housing 12 and the porous film 13, oxidation of the plating solution can be suppressed. Therefore, the oxidation inhibitor does not necessarily need to be added to the electroless plating solution. By sealing the plating solution by the housing 12 and the porous film 13, hydrogen is easily made eutectoid in the plating film, thereby ensuring the improvement of solder wettability.

The metal substrate B may be a polymer resin, such as an epoxy resin, or a metal layer formed on a surface of a ceramic and the like. A resist pattern may be formed on the surface of the metal substrate B.

The following describes the method for forming the plating film using the film formation device 1 according to the embodiment. FIG. 2 is a schematic cross-sectional view for describing a state of the film formation device 1 illustrated in FIG. 1 during the film formation.

First, as illustrated in FIG. 1, the metal substrate B is disposed on the placement table 40 so as to be opposed to the porous film 13. Subsequently, as illustrated in FIG. 2, the pressing unit 18 is used to move the housing 12 downward toward the placement table 40, and the porous film 13 is pressed onto the surface of the metal substrate B. By pressing the porous film 13 onto the surface of the metal substrate B, the electroless plating solution L seeps from the porous film 13 to the metal substrate B. The metal ions in the seeped electroless plating solution are reduced on the surface of the metal substrate B, and the metal plating film is formed. Thus, the metal plating film can be formed on the metal substrate B.

The above-described step can be repeatedly performed for each substrate.

A plating temperature (temperature of the plating solution chamber) in the electroless gold plating is, for example, 50° C. or more and 95° C. or less, and may be 60° C. or more and 90° C. or less. The respective upper limit values and the lower limit values of these numerical ranges can be freely combined as necessary to specify an appropriate range. When the plating temperature is 50° C. or more, the deposition rate of the metal plating film can be improved. When the plating temperature is 95° C. or less, decomposition of components in the plating solution can be suppressed.

A plating time is, for example, one to 60 minutes while it depends on the plating temperature.

EXAMPLES

While the following describes the embodiment with examples, this disclosure is not limited to these examples.

Example 1 (Porous Film Having Anionic Group)

As the porous film used in the examples, the solid electrolyte membrane (Nafion (registered trademark) manufactured by DuPont, equivalent weight: 879 g/mol, water contact angle: 10°) was used.

(Gold Plating Solution)

As the electroless plating solution, the gold plating solution (neutral cyanide-free substitution gold plating solution, EPITHAS (registered trademark) TDS-25, manufactured by C. Uyemura & Co., Ltd.) was used. The gold plating solution contains gold sodium sulfite as the metal salt, sodium sulfite and EDTA salt as the complexing agent, and phosphate as the pH buffer. The pH of the gold plating solution was set to 7.3, and a gold concentration was set to 1.5 g/L.

(Metal Substrate)

A nickel film formed on a copper block by a solid electrolyte deposition was used as the metal substrate. A polyimide tape was attached to the nickel film to form an opening of 1 cm×2 cm, and the opening was used as a film formation region. In the solid electrolyte deposition for forming the nickel film, a foam nickel (23 cm×23 cm×0.2 cm, NI318201, manufactured by Nilaco Corporation) was used as a positive electrode, and various conditions were set as follows. Pressing force: 1 MPa, temperature: 60° C., current: 150 mA, film formation period: 400 seconds, film thickness: 10 μm, and film formation area: 1×2 cm.

(Plating Film Formation)

The electroless plating was performed using the film formation device 1 that has the configuration illustrated in FIG. 1 and includes the solid electrolyte membrane as the porous film 13. First, the plating solution chamber 15 was filled with the gold plating solution, and the gold plating solution was impregnated to the solid electrolyte membrane. The temperature of the gold plating solution was set to 75° C. Subsequently, the gold plating solution was dropped to the nickel film by 0.1 ml by a drop mechanism (not illustrated) included in the film formation device 1. Subsequently, the solid electrolyte membrane was pressed onto the nickel film by the pressing unit 18, thereby forming the gold plating film on the nickel film. The pressure was set to 1 MPa, and the film formation period was set to 30 minutes. The film thickness of the gold plating film was approximately 0.05 μm.

Comparative Example 1

The gold plating film was formed by the method similar to that of Example 1 excluding that a polyethylene porous film (SETELA (registered trademark), manufactured by Toray Industries, Inc.) was used instead of the solid electrolyte membrane.

[Evaluation and Result]

Appearances of the obtained gold plating films were observed with a microscope (VH-8000, KEYENCE CORPORATION). As a result, it was confirmed that a uniform gold plating film was obtained in Example 1. Meanwhile, in Comparative Example 1, the gold plating film adhered to the polyethylene porous film, and the gold plating film peeled off from the nickel film when the porous film was pulled up. Therefore, the gold plating film could not be formed on the nickel film. Since the solid electrolyte membrane used in Example 1 has the anionic group, its surface is hydrophilic. Therefore, the wettability of the gold plating solution is high, thus uniformly spreading the gold plating solution on the surface of the nickel film at the pressing. Consequently, it is considered that the uniform gold plating film could be obtained. Meanwhile, the polyethylene porous film used in Comparative Example 1 has no functional group, and the wettability of the gold plating solution is low. Therefore, it is considered that the gold plating solution could not be uniformly spread on the surface of the nickel film at the pressing but adhered to the nickel film, thus failing to form the gold plating film.

Example 2

Subsequently to Example 1, the gold plating film was continuously formed 60 times. While the gold plating solution was dropped in Example 1, the gold plating solution was not dropped in this example. In the continuous formation of the gold plating film, only the metal substrate was replaced for every film formation.

Comparative Example 2

In Comparative Example 2, the gold plating film was formed on the nickel film 60 times by the conventional method. That is, the nickel film was immersed to the gold plating solution to continuously form the gold plating film 60 times. In the continuous formation of the gold plating film, only the metal substrate was replaced for every film formation. The plating conditions were set to the temperature at 75° C., the pH at 7.3, and the gold concentration at 1.5 g/L. A plating tank (glass beaker) was disposed in a water bath, and a temperature of the plating tank was controlled by an automatic temperature controller included in the water bath. The plating solution was circulated by a pump to be stirred. The stirring of the plating solution was adjusted so as to avoid direct hitting of liquid flow to the metal substrate. A rocking speed was set to 0.5 m/minute. The plating tank was not sealed. When the plating solution was stirred, a jet strength of the pump was adjusted so as to avoid entrainment of air into the plating solution. When the air was mixed in the plating solution, an anti-foam agent containing ethylene glycol as a main component was added to the plating solution.

[Evaluation and Result]

For Example 2 and Comparative Example 2, surface roughness (Ra and Rz) of the gold plating film at the first time and the gold plating film at the 60th time were each measured. The appearances of the gold plating film at the first time and the gold plating film at the 60th time were each observed with the microscope (VH-8000, KEYENCE CORPORATION). Table 1 indicates the result.

As indicated in table 1, in Example 2, significant change was not seen in the surface roughness or the appearance of the gold plating film even after the film formation of 60 times.

Meanwhile, in Comparative Example 2, after the film formation of 60 times, the surface roughness of the gold plating film increased and the appearance also became non-uniform. This is considered that since nickel ions were dissolved from the nickel film to make the gold plating solution unstable, parts on which the film formation was not performed were generated to make the surface shape non-uniform.

TABLE 1 Surface Surface Appearance Roughness (Ra) Roughness (Rz) (×450) First 60th First 60th First 60th Time Time Time Time Time Time Example 2 0.08 0.09 0.8 1.0 Uniform Uniform Compara- 0.09 0.15 0.9 1.8 Uniform Non- tive Ex- Uniform ample 2

Example 3

This example examined the change in coverage of the gold plating film when the equivalent weight (EW) of the solid electrolyte membrane was changed.

In the measurement of the equivalent weight, the solid electrolyte membrane to be measured was precisely measured (=a gram) in a sealable glass container, an excessive amount of calcium chloride aqueous solution was added there, and stirring was performed for 12 hours. Hydrogen chloride generated in the system was titrated (b [ml]) with a 0.05N sodium hydroxide aqueous solution (titer f) using phenolphthalein as an indicator. From the above-described measurement values, an ion exchange equivalent weight (g/mol) was obtained using [Formula 1].

ion exchange equivalent weight [g/mol]=(1000/a)/(0.05×b×f)  [Formula 1]

Table 2 indicates the equivalent weights of the manufactured eight kinds of solid electrolyte membrane.

Subsequently, the manufactured eight kinds of solid electrolyte membrane were used to form the gold plating films by the method similar to that of Example 1.

[Evaluation and Result]

For the obtained eight gold plating films, the coverage was calculated. Table 2 and FIG. 3 indicates the result.

TABLE 2 Equivalent Weight (EW) Coverage Example 3A 735 40 Example 3B 812 60 Example 3C 874 90 Example 3D 879 100 Example 3E 909 95 Example 3F 946 75 Example 3G 1000 70 Example 3H 1100 60

It was confirmed that the coverage changed depending on the equivalent weight (EW) of the solid electrolyte membrane. From the result indicated in Table 2 and FIG. 3, it was confirmed that the coverage was improved when the equivalent weight (EW) was 850 to 950 g/mol. It was confirmed that the range of the equivalent weight was 874 g/mol or more and 909 g/mol or less in some embodiments.

Claims following the described disclosure are explicitly included in the described disclosure in the specification, and the claims are each independent as an individual embodiment. This disclosure includes all replacements of the independent claims with their dependent claims. Furthermore, additional embodiments derived from the independent claims and the following dependent claims are also explicitly included in the described specification.

Those skilled in the art can use the above-described description for the fullest extent of the present disclosure. The claims and embodiments disclosed herein are merely illustrative and exemplary, and should not be construed as limiting the scope of the disclosure in any way. With the help of this disclosure, changes may be made in the details of the above-described embodiments without departing from the underlying principles of the disclosure. In other words, various modifications and improvements of the embodiments specifically disclosed in the above specification are within the scope of the present disclosure. 

What is claimed is:
 1. A method for forming a metal plating film on a metal substrate by a substitution-type electroless plating method, the method comprising bringing a porous film into contact with a surface of the metal substrate, the porous film having an anionic group and containing an electroless plating solution.
 2. The method according to claim 1, wherein the bringing includes reducing metal ions derived from the electroless plating solution contained in the porous film to deposit the metal plating film on the surface of the metal substrate.
 3. The method according to claim 1, wherein the anionic group comprises at least one kind selected from the group consisting of sulfonate group, thiosulfonate group, carboxy group, phosphate group, phosphonate group, hydroxy group, cyano group, or thiocyano group.
 4. The method according to claim 1, wherein the porous film is a solid electrolyte membrane having an ionic conductivity.
 5. The method according to claim 4, wherein the solid electrolyte membrane comprises a fluorine-based resin having sulfonate group.
 6. The method according to claim 5, wherein the solid electrolyte membrane has an equivalent weight (EW) of 850 to 950 g/mol.
 7. The method according to claim 1, wherein the electroless plating solution comprises an electroless gold plating solution.
 8. The method according to claim 7, wherein the electroless gold plating solution comprises at least a gold compound and a complexing agent.
 9. The method according to claim 8, wherein the gold compound comprises a non-cyanide gold salt, and the complexing agent comprises a non-cyanide complexing agent.
 10. The method according to claim 9, wherein the non-cyanide gold salt comprises a gold sulfite salt.
 11. The method according to claim 9, wherein the non-cyanide complexing agent comprises a sulfite salt.
 12. The method according to claim 10, wherein the anionic group comprises sulfonate group.
 13. The method according to claim 9, wherein the non-cyanide complexing agent comprises mercaptosuccinic acid.
 14. The method according to claim 13, wherein the anionic group comprises carboxy group.
 15. The method according to claim 1, wherein the metal substrate comprises nickel or a nickel alloy.
 16. The method according to claim 1, wherein the electroless plating solution is supplied from a plating solution chamber that houses the electroless plating solution.
 17. The method according to claim 16, wherein the plating solution chamber is disposed in contact with the porous film.
 18. A film formation device for forming a metal plating film on a metal substrate by a substitution-type electroless plating method, the film formation device comprising: a porous film having an anionic group; a plating solution chamber disposed in contact with the porous film, the plating solution chamber housing an electroless plating solution; and a pressing unit that brings the porous film into contact with the metal substrate by relatively pressing the plating solution chamber and the metal substrate. 